Intuitively this seems wrong to me. If the satellite is overhead, the error would put you 300m into the ground so to speak. I'm not sure why you project that horizontally, and especially why you take the distance to the satellite into account.
As another sanity check, if the error for 1 us is 110 km, the error for 1 ns would be 110 m, and I suspect 1 ns error is not unusual for consumer electronics:
> To reduce this error level to the order of meters would require an atomic clock. However, not only is this impracticable for consumer GPS devices, the GPS satellites are only accurate to about 10 nano seconds (in which time a signal would travel 3m)